During processing of silver halide color photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a "fixing" step. In some processes, the two steps can be combined in a so-called bleach-fix step.
Common bleaching agents include ferric chelate complexes of aminopolycarboxylate ligands, such as ethylenediaminetetraacetic acid (EDTA) and 1,3-propylenediaminetetraacetic acid (PDTA). These agents perform acceptably, but are not generally biodegradable, and environmental concerns are very prominent in many cultures.
Other bleaching agents are known which have one or more deficiencies. For example, ferric complexes of .beta.-alaninediacetic acid are known, but they are relatively slow bleaching agents compared to the ferric-EDTA complexes. Thus, they must be used in higher concentrations which is undesirable for cost and environmental reasons.
Japanese Kokai 51-07930 (published Jan. 22, 1976) describes the use of nitrilotriacetic acid or 2,6-pyridinedicarboxylic acid or both to reduce stains in neutralizing or fixing solutions. Bleaching solutions containing an aminocarboxylic acid metal complex salt or a polycarboxylic acid metal complex salt are also known. Japanese Kokai 53-048527 (published May 2, 1978) describes the use of such complexes to lower fog.
EP-A-0 329 088 (published Aug. 23, 1989) describes bidentate complexes in bleaching solutions which further contain buffers, one of which is 2-pyridinecarboxylic acid (PCA). Complexes of PCA with iron are not described.
Other biodegradable bleaching agents, such as ferric citrate, are effective only at very low pH, such as below pH 3 (see for example, DE 3,919,551A1). Another example of a biodegradable bleaching agent is the ferric complex of 2,6-pyridinedicarboxylic acid (PDCA). This complex has been demonstrated as an efficient catalyst for persulfate bleaching, as described in copending and commonly assigned U.S. Ser. No. 07/990,500 (filed Dec. 14, 1992 by Buchanan et al now abandoned). However, the ferric complex is insufficiently water-soluble to be used at the concentrations required for a commercially viable bleach in which the ferric complex is the primary oxidant.
Bleaching solutions have been developed which contain more than one ligand and which help provide rapid bleaching without unwanted dye formation in color photographic materials. However, such solutions contain two distinct iron-complex salts. For example, in KODAK FLEXICOLOR.TM. Bleach II, one salt is ferric ammonium-EDTA, and the other is ferric ammonium-PDTA. While such mixtures are stable and provide excellent bleaching, neither of the noted complexes is readily biodegradable. Other mixtures of complexes are described in EP-A-0 430 000, but they lack stability when used in combination with thiosulfate fixing agents. Other ligand mixtures are described in EP-A-0 534 086 wherein bidentate ligands are used as buffering agents.
Useful ternary bleaching agents are described in copending and commonly assigned U.S. Ser. No. 08/128,626 (filed Sep. 28, 1993 by Gordon et al pending). Such materials comprise one iron atom and two different ligands. While these materials are useful in some processes, there continues to be a need for more rapid processes using biodegradable materials.
Japanese Kokai 50-26542 (published Mar. 19, 1975) describes bleaching solutions containing an iron chelate with one or more ligands such as 2-carboxypyridine, 8-hydroxyquinoline or 2-carboxypyrazine. However, the mol ratios of these ligands to iron are quite low as demonstrated in the examples of that publication. Such ratios fail to provide the rapid and superior bleaching performance desired in the industry.
There remains a need in the art for highly water-soluble bleaching agents which preferably comprise biodegradable ligands, provide rapid bleaching and are compatible with chloride rehalogenation.